1. Field of the Invention
This invention relates generally to subterranean instrumentation and telemetry systems and relates more specifically to apparatus for logging wells by remote sensing and real-time surface recording of well drilling parameters.
2. Description of the Prior Art
During the drilling of a well, certain parameters concerning the drilling operation and the earth strata being drilled through are of interest. Knowledge of the materials comprising the earth strata is valuable to the drilling rig operators. This knowledge enables the weight, speed, and torque of the drill bit to be adjusted to obtain optimum drilling performance. Knowledge of the strata also permits a suitable selection of a drilling fluid, which is pumped down the hollow drill pipe to convey the drill tailings to the surface, and to keep the hydrostatic pressures in balance.
It is desireable to identify the strata beneath the drill bit, prior to its contamination by the drilling fluid, as well as the strata that has been drilled through. Information concerning the drill bit, such as torque and weight on bit, is of interest and can be utilized to optimize drilling performance. Knowledge of certain drilling fluid characteristics, such as temperature and pressure, is useful as well.
All of this information is most useful and most indicative of the drilling environment when it is obtained while in the process of drilling. Parameters concerning the drilling operation should be measured in the dynamic drilling environment and relayed in real-time to the surface to permit the drilling rig operators to optimize drilling performance. Identification of the earth strata while drilling is also desireable. One common method of strata identification requires the drilling to be halted and the drill pipe and drill bit to be completely removed from the bore hold to allow a wire-line sensor to be lowered into the well for strata identification. By this time, the strata has been contaminated by the drilling fluid which alters its resistivity and makes it more difficult to identify and evaluate. Then, when the measurements are completed, the sensor is removed from the bore hole and the drill pipe and drill bit are replaced. This very time consuming and expensive procedure would be unnecessary if the strata could be identified and evaluated while drilling.
Several problems are encountered in logging wells, in both the measurement of the parameters of interest and in the communication of these measurements to the top of the well. The down-hole environment itself is quite harsh with elevated temperatures and pressures. Drill bit vibrations may be quite high. The drilling fluid flowing through the drill pipe bit may be highly abrasive. One design consideration is that the down-hole measurement unit must be durable enough to withstand this hostile environment for long periods of time. Another design consideration is that electric power must be provided to the down-hole measurement unit for powering the measurement sensors and circuits. Additionally, data gathered by the down-hole measurement unit must be communicated, in some manner, to the surface to provide real-time measurement while drilling (MWD). The use of cables to provide electric power to the down-hole measurement unit and to provide data communication between the down-hole measurement unit and a surface recording unit are impractical with standard rotary drilling procedures. Electromagnetic signals, propagated through the earth, may be used to communicate between the down-hole measurement unit and the surface recording unit, but signal attenuation and noise problems limit the usefulness of this approach.
A substantial amount of prior art exists concerning well logging apparatus. However, only the most appropriate art will be cited herein. Toroidal transformers, which are toroidal cores of magnetic material wound with wire, are known for use in well logging apparatus. Still discloses in U.S. Pat. Nos. 3,793,632 and 4,302,757 methods of using toroids to transmit data along a drill string. Silverman, in U.S. Pat. No. 2,354,887, discloses the use of a toroidal transformer for sensing a surface generated current in a drill pipe that is proportional to the conductivity of the earth strata between the drill bit and the surface. Arps in U.S. Pat. No. 3,305,771 and Martin in U.S. Pat. No. 3,079,549, disclose the use of toroidal transformers to sense current flow in drill pipes for logging wells. See also "Theory of Transmission of Electromagnetic Waves Along a Drill Rod in Conducting Rock" by James R. Wait and David A. Hill; trans. on Geoscience Electronics, pp. 21-24, Vol. GE-17, No. 2, April, 1979.
The use of toroidal transformers for inducing a modulated alternating current indicative of logging data in a drill pipe at a subsurface location for transmission to the surface is disclosed by Silverman in U.S. Pat. Nos. 2,354,887 and 2,411,696, by Scherbatskoy in U.S. Pat. No. 4,057,781, and by Zuvela in U.S. Pat. No. 4,181,014. Scherbatskoy, in U.S. Pat. No. 4,057,781, also discloses a toroidal transformer employed as a device for sensing modulated alternating current in a drill pipe at the surface of a well as part of a circuit for receiving transmitted logging data. Silverman, in U.S. Pat. No. 2,411,696, discloses a similarly functioning toroidal transformer that is located below the surface for reduced noise sensitivity. However, none of the above cited patents disclose the use of a toroidal transformer located in close proximity to a drill bit for inducing a current through the drill bit in order to measure the conductivity of the surrounding earth strata.
A substantial problem associated with the use of a drill pipe as a communication medium is the signal loss which occurs along the length of the drill pipe due to conductive contact with either the earth strata surrounding the borehole or the drilling fluid surrounding the pipe. Heretofore, others have attempted to overcome this problem in various manners. For example, in U.S. Pat. No. 2,153,802 Jakosky discloses the use of an insulative drilling fluid and insulative spacers attached to the drill pipe to prevent the drill pipe from contacting the borehole, and Williston in U.S. Pat. No. 2,755,067 discloses the use of an insulative coating which is sprayed upon the surface of the drill pipe. However, neither of these techniques has been found to be effective in substantially reducing transmission losses along the drill pipe in deep hole applications.
A down-hole electrical generator powered by fluid flow is of interest in the present invention. One such generator is disclosed by Godbey in U.S. Pat. No. 3,305,825.
Alternative electrical and electromagnetic means for transmitting logging data from an area near a drill bit to the surface of a well are known in the art of well logging. See, for example, U.S. Pat. Nos. 2,181,601 (Jakosky), 3,967,201 (Borden), and 4,087,781 (Grossi) for various alternatives.